Well it's only been said about 10 times here but I also think you should go for it if you have the $$. I think it is more important to get another gig of memory though, this may stop the computer from accessing your harddrive (much faster when it isnt)
The X1800XT will be fine, as long as you back it up with 2 gigs of RAM. If you really have to, then go buy the X1900XT, but also do make sure you have that two gig of RAM, Oblivion will love you for it. And a 485W PSU should handle it fine.... as long as it isn't a generic PSU, because unbranded PSU suck major cockchops...
And even in the games where there is a difference, it's minor.
For the most part moving from 512 to 1GB is a huge improvement while moving from 1 to 2 is minor at best, and nothing at worst (well actually I saw a few that went down, but by the marginn of error so doubt it's detrimental.
And I say that as someone who has 2GB in my laptop, why, because I will move it into my next rig when it may be helpful and cost about $30CDN more than 1GB so why not?
But few people need it, and it's definitely not an impactful upgrade for Oblivion.
alot with the ram is how you tweak windows, if youre running on default and leave all your apps going during games you will need alot of ram.
When they bench it's a clean install with only the bare drivers and no other apps running, they don't acount for a dirty system. If you set win to performance and keep your system clean,defraged and turn off all apps but the game one gig is all you really need. If you just install a game and hit the icon to play, you have half your ram filled with junk[ virus scans, pop up blocker and the like]. Every thing wants to run in ram now even if you don't need it 90% of time, sloppy coding really. I would also defrag before and after you install a large program. So many people blame the vid card when it's really the harddrive being read because the ram is full.
I'm sure vista will be a sloppy ass ram hog so two gigs is a good idea. I was hoping MS would take a hint from linux and make a smarter OS but it doesnt look like it. With all the flavors of vista one should be gaming, but isn't
485watts is more than enough....check the juice on 12volts rail..should be between 20 amps and 30 amps..or even more
although nobady said x1900 should need more power than x1800
but of course is good practice to have a good psu.....that will ensure you a good and stable sistem for a long time